1. Field of the Invention
This invention relates to crystal oscillators, and, more particularly, to crystal oscillators providing low distortion sinusoidal output signals.
2. Prior Art
FIG. 1 shows a series feedback crystal oscillator circuit 10 which includes a noninverting amplifier 12 and a crystal 14 in the positive feedback path. The crystal has low impedance at series resonance frequency and high impedance at other frequencies, so the feedback signal is greatest at series resonance. If the amplifier 12 has sufficient gain, the circuit 10 oscillates at a frequency near the crystal resonant frequency. Total loop gain for oscillation, including the effect of the crystal impedance, must be precisely 1. Additional gain causes the amplitude of the oscillation signal to grow until the amplifier begins to clip the signal or to saturate causing the time-averaged gain of the current to drop to 1, at which point the oscillator operation is stable.
Many oscillator circuits are designed to allow the amplifier to clip the oscillation signal at predetermined levels. This design approach produces poor amplitude control for the signals at the crystal input terminal, since the clipping levels or saturation characteristics of a saturating element often vary with time, temperature, power supply levels, and with other environmental factors. A crystal input terminal is a poor place to attempt to obtain amplitude control because the physical amplitude of the oscillation signal is best measured in terms of the output current of the crystal, not its input voltage. The resonant frequency of the crystal is a function of the amplitude of the oscillation signal so it is more important to have control of the amplitude of the crystal output signal instead of the crystal input signal.
Prior art designs for oscillator circuits have produced large harmonic-component signals due to clipping of the oscillation signal. These harmonics may cause interference to other circuits. Harmonic signals pass through the shunt capacitor C.sub.o of the crystal, which can cause problems with the oscillator amplifier and make it difficult to cancel the effect of C.sub.o. An additional effect of non-linear operation is that the phase shift though the oscillator circuit cannot be accurately predicted. The phase shift depends on the amount of time that a saturated component takes to recover from saturation. Recovery time can vary greatly depending on environmental conditions. The unpredictability of phase shift is especially important when using low Q crystals which produce significant changes in frequency for small changes in phase shift.
A non-crystal controlled prior art sinewave oscillator used as a waveform generator is disclosed in a 1976 National Semiconductor Company publication entitled "Linear Applications," Volume 1, AN72-19. The frequency selective element is formed by two RC active filter stages. The average value of the sinewave output voltage is detected. The output level is regulated by comparing the average value of the sinewave to a DC reference voltage to provide a sinewave output signal using a differential averaging circuit.